Corrosion and impact-resistant article



Sept. 25, 1956 P. P. TURNER, JR., ET AL 2,763,920 CORROSION AND IMPACT-RESISTANT ARTICLE Filed March 6, 1951 'Illill/llllllll,

ZE-:VE: :T1/fm TRE Percy P Turner* 1:77? Poyef Puppenaer 777. HZ-/E United States Patent O 2,763,920 connosioN MACT-RESISTANT ARTICLE Percy l. Turner, lr., Euclid, and Robert R. Ruppender, Cleveland, Chio, assignors to Thompson Products, Inc., Cleveland, Ohio, a corporation of Ohio Application March 6, 19'51, Serial No. 214,116 4 Claims. (Cl. 29-198) The present invention relates to an impact and corrosion-resistant article particularly useful in environments of high temperatures and corrosive atmospheres. The present invention is also concerned with methods of manu.- facturing such impact and corrosion-resistant articles.

The extensive development in the field of jet engines has necessitated the development of alloys for usein' the manufacture of parts for such jet engines which canrwithstand the extremely high temperatures Vand the oxidizing atmospheres normally present in the operation of turb o jet engines. VFor example, a turbine bucket attached to a turbine wheel will ordinarily be exposed to temperatures in excess of about 1450o F. and in order to function properly, it must have a high degree of strength, toughness, creep-resistance, and resistance to the oxidizing gases present in the turbine engine.

In addition to their use in the manufacture of turbine buckets, articles produced in accordance with the present invention may also be employed under conditionsof higher temperature and lower stress than are encountered by turbine buckets. One such application occurs in nozzle diaphragm vanes in gas turbines which must withstand very severe conditions of temperature and thermal shock, but at a relatively low stress. v v

Refractory metals such as molybdenum and tungsten exhibit excellent properties of strength, toughnessl and creepfresistance at elevated temperatures. However, the oxidation resistance of. metals such as molybdenum is quite poor. Although molybdenum has a melting point in excess of 4500 F. and good strength at elevated temperatures, it begins oxidation at temperatures as low as 900 F. The rate of formation of the molybdenum oxide increases with the temperature. Since the molybdenum oxide produced under these oxidizing conditions sublimes, complete disintegration of the molybdenum body Willoccur in a relatively short time under such high temperature oxidation conditions.

it will be understood that the Vremarks applicable .to molybdenum are applicable also to molybdenum base alloys, particularly those containing 95% o r more by weight molybdenum. The present invention is directed both to the use, of substantially pure molybdenum bodies and the molybdenum base alloys mentioned.

it has recently been suggested that the oxidation resistance'of a molybdenum article can be increased, while still preserving its high strength properties, by depositing certain metals, or metalloids on the surface of thermolybdenuin. Certain metals havevthe ability to form intermetallic compounds with the molybdenumy surface and these intermetallic compounds have been found tobe very corrosion-resistant. Typical of the coating materials are elements such as silicon, aluminum, andzirconium. rf'he intermetallic compounds formed between these elements and molybdenumform an integral bond with the molybdenum surface. When the molybdenum body is provided with such intermetallic compound corrosionresistant layers, the coated molybdenum body can withstand thousands of hours of operation above temperatures 2,763,920 Patented Senf 2.5, :1.9.5.6

2 ofred heat without `showing any evidence of oxidation Vof ,the molybdenum base.

While the above described oxidation resisting coatings very effectively protect the surface of the refractory ,metal from oxidation, they are subject to the .disadvantage vvthat the intermetallic compounds formed vat the surface ofthe refractory body are quite brittle, and liable to failure due to impact. Such impact could result during the operation `of gas turbine engines in aircraft due .tothe presence of foreign objects, even small particles, in the gas streams of the engine. These foreign Objects may be received from the outside air, or may result from chipping of the combustion tubes, or from a number of other sources. These particles pass through the turbine at extremely high velocities, on the order of several hundred feet a second. When the objects strike the coated turbine blade, fracture of the coated surface often results, thereby exposing the refractory metal body to the corrosive atmosphere o f the gas stream.

In the .present invention, brittle, corrosion-resistant articles are provided with an impact resistant coating comf prising a ductile nickel-chromium alloy, preferably containing boron, the coating being integrally bonded to the Underlying brittle, .Corrosion-.resistant surface; or alternatively, directly bonded to a molbdenum surface,

An object of the present invention is -to provide a corrosisaand .impact-resistant .article .having the. ability t0 withstand corrosive atmospheres normally present in gas .turbine engines, .and beine resistant to fracture by high velsity Particles. in .the aas stream.-

Another object of the present invention is to provide an :insect-.resistant matins for refrasfory metal. bediesrwhich have a Surface. of brittle., @assise-resistant material:

. Still another objectief the present invention is to proavidea` coated .refractory metal suitable for'useuinmanu.- factnre of parts for lturbo-jet engines, and the like. Another object of the present inventionis to .provide a method for bonding an impact-resistant alloyllayer onto the brittle surface of a `cporrosionfresis,tant article. The present invention is concerned with Vproviding brittle, corrosion-resistant surfaces such as molybdenum disilicide or molybdenum metal itself with an impact-re,- sistant coating containing a nickelchromium alloy. In a preferred embodiment of the invention, boron is included in the nikelfchromium alloy to lower-the melting point andredluce the surface tension of the alloy thereby inf creasing its iluidity and free flowing characteristics. Such coatings have been found .to be extremely resistantv to im.- pact and serve .to protect .the underlying corrosion re: sisting layer from fracture by high velocity particles..

The brittle, ycorrosion-resistant surfaces can be .applied to the molybdenum or tungsten .base in. -a variety of methods which involve no. Vpartof the instant invent-ion, Porexample, the preferred method of coating molybf denum .with .silicon .involves the deposition of silicon in vapor phase on aheated molybdenum body maintained at a temperature between about 1600 F. 'and 2 3,00 P, in an atmosphere of hydrogen.. The silicon may be in.- ,tlroduced in the reaction chamber in the form of vaporized silicon tetrachloride, which decomposes under the condif tion in the reaction zone to yield free silicon. The liberated silicon Vreadily combines with the surfaces of molybdenum to form intermetallic layers of molyb-A denum-silicon compounds. Coated molybdenum articles produced according to this4 process evidence a silicon concentration gradient at themsurfaces thereof. Forl e x. ample, an intermetallic layer isV ordinarily produced next to the molybdenum base metal consisting primarily of molybdenum-silicon compounds which have relatively low concentration of silicon, `such as molybdenum monosilicide. The next layer overlying the molybdenum monosilicide layer contains molvbdenum-silicon com- 2,7es,92o Y pounds having higher silicon concentrations such as molybdenum disilicide. It is the molybdenum disilicide layer which is believed to contribute the most effective ,corrosion resistance to the article and it is this layer that needs to be protected against fracture by impact. However, the coating alloy of this invention has substantial corrosion resistance, and can be applied directly to a molybdenum article without the intermediate layer.

The nickel-chromium alloy which is used to provide the impact-resistant coating may contain varying amounts of nickel and chromium and preferably includes at least a small percentage of boron.

The following table illustrates the range of composition of the alloys which may be employed in the practice of the present invention and two specific alloy compositions:

The balance of the composition of the typical alloys listed consists of the usual impurities.

The amount of carbon in the alloy is an important consideration, as carbon tends to combine with chromium to form chromium carbide, thereby reducing the effective chromium content. The reduction in elective chromium content reduces the oxidation resistance of the alloy at high temperatures.

The silicon may be added as ferro-silicon, and serves to increase the high temperature resistance to oxidation.

Iron imparts fluidity to the alloy and renders it more workable. This metal may be added in the form of ferro-silicon, and it is also present in some of the other ingredients as an impurity.

f The' boron, as previously described, not only lowers the melting point of the alloy but increases the fluidity of the molten alloy by decreasing its surface tension.

Manganese occurs as an unavoidable impurity and lowers the high temperature resistance of the alloy. The rst tenths of one percent are the most detrimental in this respect.

Calcium and zirconium are added to the alloy to act as deoxidizing agents. The more completely deoxidized the alloy is, the less susceptible it is to precipitation of oxides at the grain boundaries of the alloy. These two elements therefore serve to increase the high temperature resistance of the alloy.

Prior to coating the corrosion-resistant article with the nickel-chromium alloy it is very desirable to clean the surface of the molybdenum disilicide, or other corrosionresistant coating to secure a better adhesion of the nickelchromium alloy. This treatment can be carried out by first degreasing the article in a solvent such as trichloroethylene vapor. After degreasing, the surface of the coated molybdenum body is etched with a reagent capable of at least partially dissolving the silica, silicon and/or silicon rich phases present. One such reagent is an aqueous solution of hydrofluoric acid containing one part by volume of 40% to 50% hydrouoric acid and two parts water. The article is etched for a period from 20 to 120 seconds, with 40 seconds being suitable for most purposes. The molybdenum disilicide surface is thereby rendered more receptiveto the subsequently applied nickel-chromium alloy layer.

The process of applying the coating consists in liquifying the nickel-chromium alloy and spraying the liquilied alloy onto the surface of the article. For most applications, the thickness f this coating is preferably on the 4 order of 0.007 inch, but may extend within the range of about 0.002 inch to 0.020 inch, or even higher. Alternatively, the powdered alloy may be suspended in a liquid vehicle and sprayed on the surfaces to be coated.

After spraying, the coated article is then heated in a non-oxidizing atmosphere to braze the coating onto the corrosion-resistant surface. This brazing operation may be carried out in a dry hydrogen atmosphere, or under vacuum conditions, for a period up to 120 minutes at temperatures from about 1800 to 2300 F.

Other methods for applying the coating include dipping the article to be coated in a molten alloy bath, spraying the powdered alloy through a hot flame, or feeding the alloy as a rod into a llame and spraying the surface with the liquied metal.

The attached sheet of drawings illustrates more specifically the structure of a coated molybdenum body produced in accordance with the present invention.

In the drawings:

Figure 1 is a view in elevation of a turbine bucket provided with a coating according to the present invention;

Figure 2 is a fragmentary cross-sectional view, greatly enlarged, taken substantially along the line I l-II of Figure l; and

Figure 3 is a view similar to Figure 2 and illustrates the effect produced when the coated article is struck with a high velocity particle.

As shown on the drawings:

Reference 10 denotes generally a conventional turbine bucket consisting of a blade portion 11 and a r-tree root portion 12 arranged for wedged engagement along the hub of a turbine wheel.

As illustrated in the greatly enlarged view of Figure 2, the bucket 10 consists of a body of molybdenum metal 13 having a siliconized corrosion-resistant outer surface. The surface nearest the molybdenum body, designated at 14, contains intermetallic compounds of molybdenum and silicon having a relatively low percentage of silicon, such as molybdenum monosilicide. The next adjoining layer 15 contains relatively brittle crystals of compounds containing higher percentages of silicon, such as molybdenum disilicide.

The outermost layer which provides the impact resistance to the molybdenum disilicide layer 15 consists of a nickel-chromium alloy containing boron, as previously described. This layer has been designated generally at numeral 16. In operation, the nickel-chromium layer will be subjected to oxidation with the formation of a thin oxide film 17 which further aids in preventing diffusion of oxygen into the inner layers of the body.

Figure 3 illustrates the condition where the structure of Figure 2 is struck with a high velocity particle. The

' nickel-chromium layer 16 is sufficiently tough so that the per second. Four such pellets were caused to strike a specimen 0.094 inch thick at points one quarter of an inch apart. The series of four pellets dented the coating, but did not fracture the same. After this ballistics test, the specimen was subjected to oxidation at a temperature of 1600 F. without evidence of failure.

From the foregoing it will be appreciated that we have herein provided a novel type of corrosion and impact resistant material particularly suited for use in the manufacture of parts for jet engines. The impact-resistant coatings have the ability to become securely bonded to molybdenum or brittle corrosion-resistant layers containing molybdenum and serve as a very effective shock absorbing layer for the underlying surfaces. The coatings can be applied to selected surface areas of a turbine bucket; or the entire bucket, including the airfoil and root portions, may be coated.

It will be understood that modications and variations may be effected without `departing from the scope of the novel concepts of the present invention.

We claim as our invention:

1. An impact and corrosion resistant turbine bucket comprising a base of molybdenum metal and an impact resistant coating overlying and bonded to said molybdenum base, said coating comprising a nickel-chromium alloy consisting essentially of from about 65 to 90% nickel, from about 8 to 20% chromium, from about 1.0 to 8.0% boron with the balance consisting essentially of iron and silicon, said coating being capable of withstanding mechanical shock without fracturing.

2. An impact and corrosion resistant turbine bucket comprising a base of molybdenum metal and an impact resistant coating overlying and bonded to sai-d molybdenum base, said coating comprising a nickel-chromium alloy consisting essentially of from about 65 to 90% nickel, from about 8 to 20% chromium from about 1.0 to 8.0% boron, from about 1.0 to 5.5% iron and from about 0.5 to 5.0% silicon, said coating being capable of withstanding mechanical shock Without fracturing.

3. An impact and corrosion resistant turbine bucket comprising a base of molybdenum metal and an impact resistant coating overlying and bonded to said molybdenum base, said coating comprising a nickel-chromium alloy consisting essentially of from about 65 to 90% nickel, from about 8 to 20% chromium, from about 1.0 to 8.0% boron, from about 1.0 to 5.5 iron, from about 0.5 to 5.0% silicon and less than 0.7% carbon, said coating being capable of withstanding mechanical shock without fracturing.

4. An impact and corrosion-resistant turbine bucket comprising a base of molybdenum metal, a corrosionresistant coating of molybdenum disilicide bonded to said molybdenum base, and an impact-resistant coating bonded to and overlying said molybdenum disilicide coating7 said impact-resistant coating consisting essentially of a nickelchromium alloy consisting essentially of to 90% nickel, 8 to 20% chromium, and from 1 to 8% boron with the balance substantially iron and silicon and capable of withstanding mechanical shock without fracturing.

References Cited in the le of this patent UNITED STATES PATENTS 1,180,614 Simpson Apr. 25, 1916 1,228,194 Fahrenwald May 29, 1917 1,504,736 Brown Aug. 12, 1924 1,718,563 Kelley June 25, 1929 1,807,554 Rohn May 26, 1931 1,853,370 Marshall Apr. 12, 1932 1,899,569 Howe Feb. 28, 1933 2,105,552 Rucben Ian. 18, 1938 2,162,253 Grossman lune 13, 1939 2,300,400 AXline Nov. 3, 1942 2,304,259 Karrer Dec. 8, 1942 2,304,297 Anton Dec. 8, 1942 2,352,230 Spencer June 27, 1944 2,361,962 Ronay Nov. 7, 1944 2,375,154 Volterra May 1, 1945 2,387,903 Hensel Oct. 30, 1945 2,391,456 Hensel Dec. 25, 1945 2,490,543 Roberston Dec. 6, 1949 2,491,284 Sears Dec. 13, 1949 2,588,421 Shepard Mar. 11, 1952 2,683,305 Goetzel July 13, 1954 2,690,409 Wainer Sept. 28, 1954 

1. AN IMPACT AND CORROSION RESISTANT TURBINE BUCKET COMPRISING A BASE OF MOLYBDENUM METALI AND AN IMPACT RESISTANT COATING OVERLYING AND BONDED TO SAID MOLYBDENUM BASE, SAID COATING COMPRISING A NICKEL-CHROMIUM ALLOU CONSISTING ESSENTIALLY OF FROM ABOUT 65 TO 90% NICKEL, FROM ABOUT 8 TO 20% CHROMIUM, FROM ABOUT 1.0 TO 8.0% BORON WITH THE BALANCE CONSISTING ESSENTIALLY OF IRON AND SILICON, SAID COATING BEING CAPABLE OF WITHSTANDING MECHANICAL SHOCK WITHOUT FRACTURING. 